ABSTRACT West Nile virus (WNV) is an NIAID Category B infectious zoonotic agent that causes severe neurological disease in humans and other animals. The virus and host processes that control WNV infection and immunity are not fully understood. Our preliminary studies indicate that susceptibility to WNV infection is in part regulated by innate immune defenses triggered by pathogen-associated molecular pattern (PAMP) recognition of WNV RNA by the RIG-I-like receptors (RLRs), RIG-I and MDA5. These defenses are mediated by the type I interferon (IFN) antiviral response that includes a novel IKK? signaling pathway of STAT1-serine 708 phosphorylation and the actions of specific antiviral effector interferon-stimulated genes (ISGs). Our preliminary studies also have revealed that pathogenic WNV disrupts IKK? signaling to attenuate IFN actions and evade antiviral immunity. These observations identify STAT1 serine-708 phosphorylation control as a virulence determinant of WNV infection. The proposed studies will investigate the hypothesis that WNV infection outcome is regulated by virus/host interactions that modulate RLR signaling and specific IFN-? innate immune defenses in distinct cell types. Our studies will include Aims to: (1) Define the dual role of RIG-I and MDA5 in restricting WNV infection [and modulating adaptive immunity], (2) Identify the PAMPs within the WNV RNA that are recognized by RIG-I and MDA5, (3) Define the IFN-?-mediated antiviral pathways that specifically restrict WNV infection in neurons, which are key targets of in vivo infection, and (4) Determine the mechanism of immune escape from the IKK??pathway.